1. Field of the Invention
The present invention relates to apparatus, systems and methods for reducing vortex-induced-vibrations (“VIV”), current drag, low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations. In another aspect, the present invention relates to apparatus, systems and methods comprising enhancement of VIV suppression devices for control of vortex-induced-vibrations, current drag, low frequency drift oscillations due to random waves, and low frequency wind induced resonant oscillations. In even another aspect, the present invention relates to apparatus, systems and methods comprising modified and improved performance fairings for reducing VIV, current drag, low frequency drift oscillations due to random waves, and low frequency wind-induced resonant oscillations. In still another aspect, the present invention relates to methods and apparatus for the “S-Lay” installation of pipe. In even still another aspect, the present invention relates to methods and apparatus installation of VIV suppression during the “S-Lay” installation of pipe.
2. Description of the Related Art
When a bluff body, such as a cylinder, in a fluid environment is subjected to a current in the fluid, it is possible for the body to experience vortex-induced vibrations (VIV). These vibrations are caused by oscillating hydrodynamic forces on the surface which can cause substantial vibrations of the structure, especially if the forcing frequency is at or near a structural natural frequency. The vibrations are largest in the direction transverse to flow, however, in-line vibrations can also cause stresses which are sometimes larger than those in the transverse direction.
Drilling for and/or producing hydrocarbons or the like from subterranean deposits which exist under a body of water exposes underwater drilling and production equipment to water currents and the possibility of VIV. Equipment exposed to VIV includes the smaller tubes and cables of a riser system, umbilical elements, mooring lines, anchoring tendons, marine risers, lateral pipelines, the larger underwater cylinders of the hull of a minispar or spar floating production system.
There are generally two kinds of water current induced stresses to which all the elements of a riser system are exposed. The first kind of stress as mentioned above is caused by vortex-induced alternating forces that vibrate the underwater structure in a direction perpendicular to the direction of the current. These are referred to as vortex-induced vibrations (VIV). When water flows past the structure, vortices are alternately shed from each side of the structure. This produces a fluctuating force on the structure transverse to the current. If the frequency of this harmonic load is near the resonant frequency of the structure, large vibrations transverse to the current can occur. These vibrations can, depending on the stiffness and the strength of the structure and any welds, lead to unacceptably short fatigue lives. Stresses caused by high current conditions have been known to cause structures such as risers to break apart and fall to the ocean floor.
The second type of stress is caused by drag forces which push the structure in the direction of the current due to the structure's resistance to fluid flow. The drag forces are amplified by vortex induced vibrations of the structure. For instance, a riser pipe which is vibrating due to vortex shedding will disrupt the flow of water around it more so than a stationary riser. This results in greater energy transfer from the current to the riser, and hence more drag.
Many methods have been developed to reduce vibrations of subsea structures. Some of these methods operate by modifying the boundary layer of the flow around the structure to prevent the correlation of vortex shedding along the length of the structure. Examples of such methods include the use of helical strakes around a structure, or axial rod shrouds and perforated shrouds. Other methods to reduce vibrations caused by vortex shedding from subsea structures operate by stabilization of the wake. These methods include use of fairings, wake splitters and flags.
VIV is also a common problem for subsea pipelines, especially the portions of the pipe line that span canyons or trenches on the ocean floor. These canyons or trenches can act as conduits and magnify the effects of currents at or near the ocean floor. As with vertical risers/tendons, the solution is to install VIV suppression such as fairings, wake splitters and flags.
Installation of VIV suppression after the laying of the pipe line very expensive, laborious, and dangerous. Ideally, VIV suppression would be installed on the pipe at the lay vessel as it is being laid.
There as two main methods of laying pipe, the “J-Lay” and “S-Lay.”
With “J-Lay,” a vertical lay vessel is utilized, in which pipe leaves the traveling vessel vertically, with the pipe essentially forming a “J” as it is being laid on the ocean floor. With J-Lay installation, VIV suppression is easily applied to the pipe at the vessel during installation.
With “S-Lay,” pipe leaves the lay vessel in an essentially horizontal position, and rolled off of a radially shaped “stinger” mounted aft, with the pipe essentially forming an “S” as it is being laid on the ocean floor. The stinger cross-section is a “V” shaped trough conveyor comprising a series of rollers across which the pipe passes. As the stinger is “V” shaped, only a portion of the pipe engages rollers. The problem with installing VIV during an S-Lay, is that the stinger will tend to shear off anything that extends radially from the pipe at those places where it engages the pipe.
Thus, there is a need in the art for apparatus, systems and methods for suppressing VIV and reducing drag of a marine element.
There is another need in the art for apparatus, systems and methods for suppressing VIV and reducing drag of a subsea pipeline, which can be installed during the laying of the pipeline.
There is even another need in the art for apparatus, systems and methods for laying a subsea pipeline with VIV.
These and other needs of the present invention will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.